Typically, liquid flow meters have been used to measure the flow of liquids in long distance and local distribution pipelines, process transmission lines, and delivery of product to tank trucks and railroad cars for transportation. Because of the value of the product being carried in the pipes, companies have a significant interest in being able to measure accurately the flow of these materials so that actual usage at high flow rate can be monitored and billed.
In addition, proper monitoring of a chemical process requires careful measurement of flows at various stages of the process.
CONVENTIONAL FLOW MEASUREMENT HAS BEEN ACCOMPLISHED BY BOTH MECHANICAL METERS, DOPPLER TECHNOLOGY METERS, HOT WIRE ANEMOMETERS AND CORIOLIS FORCE METER.
A typical mechanical meter operates by directing a flow of liquid past the blades of a turbine to drive a gear train that accumulates and displays the flow as a volume, usually in cubic feet. Although such meters do not directly measure precise volume, they often measure some other characteristic such as linear flow rate. Although it is the linear flow rate which is being directly measured, it is the total volume relationship to the flow which is adequate in many cases. Mechanical meters are usually expensive and require significant additional fittings and supporting hardware in order to operate properly.
Mechanical meters are problematic in several regards. First, the physical inertia of the turbine and gear train create a dampening effect which makes instantaneous measurement less responsive to fluctuations in flow. Second, mechanically actuated parts must be placed directly in the flow, which inhibits and restricts flow. Further, this results in clogging and deterioration of the mechanism due to the small amounts of oil and other contaminants that may be present in the flow and which accumulate over time. Third, there is wear of the gear train and bearings which results in a continually decreasing accuracy. Fourth, the liquid flow must be either stopped or rerouted around the turbine blades in order to service that portion of the meter residing within the liquid flow.
Some meters have been developed based upon laser Doppler technology, but these have been primarily used for wind flow measurements. Laser Doppler is a technology based on comparing the frequency shift of a pulsed electromagnetic beam reflected from a moving target to the frequency of the original pulsed beam. This frequency shift can be used to calculate both the direction and the speed of the target. This technique as applied to detect wind movement has severe limitations.
These devices require the presence of some particulate matter in the flow stream to cause backscatter of the electromagnetic beam. The presence of sufficient particulate matter in flow streams and other industrial process streams is questionable. As an example, where Doppler weather radar is employed to measure wind flow, when the air turns very cold and all moisture and particulate matter is precipitated, or "frozen out", there is no measurable Doppler velocity.
In addition, the laser beam cannot be perpendicular to the direction of flow and still detect Doppler motion. This means that for optimal detection, the Doppler beam must be as near "head-on" to the axis of flow as possible to detect maximum Doppler effect due to the velocity of the fluid. This requirement introduces problems associated with placing the detector invasively into the flow, because it is difficult to place the sensor in a pipe wall at an oblique angle.
Laser Doppler meters which use the backscatter technique experience a power reduction of 1/r.sup.4 at the receiver where r is the distance between the laser source and target. This basic physical limitation has tremendous implications on the power and size requirements of the Doppler sensor.
Finally, the detection of the Doppler effect requires Doppler analog filter banks or digital filter banks which are expensive and difficult to calibrate. The use of digital filter banks is more accurate because it eliminates the temperature variations found within analog components. However, the digital filter bank requires a sophisticated microprocessor or an Applications Specific Integrated Circuit (ASIC).
Another conventional electronic flow meter uses hot wire anemometer techniques. A thin film substrate containing a heating element and two resistors is placed intrusively in a gas flow. When current is passed through the heating element under conditions of fluid flow, the upstream resistor is cooled or maintains the temperature of the incoming flow, while the downstream resistor is heated due to the influence of the heating element. The voltage drops across the resistors are based upon their temperatures and are used by the meter's microprocessor to compute the flow against a pre-calibrated curve or table.
This technique does not work well for liquids because liquids are able to absorb such a large quantity of heat, that it is impossible to create a sufficient temperature differential with any reasonably sized heating element. Thus, hot wire anemometer devices are not suitable for a large portion of the flow meter market. In addition, a significant limitation is the required placing of the sensor intrusively in the gas flow with an energized and heated, electrical device and the attendant physical design and safety problems. Even though the power rating of the heating element is low, a safety hazard is posed if the gas is combustible and oxygen (air) is present. Again, this limits applicable marketability by eliminating its use in the natural gas or other combustible gas fields.
The Coriolis Effect is another technology upon which flow meters are based. This technology includes a manifold which diverts the flow of fluid from an inlet tube, through a loop of two parallel tubes, called flow tubes, and back into the manifold and then to a outlet tube. The two flow tubes are usually smaller than the inlet and outlet tubes, and loop in parallel over and back into the fitting. The flow tubes are vibrated at their natural frequency by an electromagnetic mechanism. This vibration sets up Coriolis forces within the flow tubes. These forces cause the two flow tubes to move away from each other. The magnitude of the separation of the flow tubes is proportional to the size of the forces generated, which is proportional to the flow rate of liquid.
There are several different means for detecting the slight separation between the flow pipes. Inductive magnetic devices may be used. However, these devices are difficult to make and thus expensive and are subject to temperature and EMF interference.
As an alternative, the placement of fiber optic cables looped between the flow pipes can be used to measure the separation of the flow pipes. This requires a light source, a three-way fiber optic beam splitter, a detector and various electronic components. The light source introduces laser or white light into a section of the fiber optic cable. This cable goes to the beam splitter which produces three output beams. One beam is used as a reference and the other two beams are introduced into two fiber cables that form two loops between the flow pipes. As the flow pipes move apart, the fiber optic loops are deformed which causes a change in the intensity of the light being transmitted along the fiber. The electronic components compare with the reference beam and the two flow pipe beams in order to calculate separation movement of the flow pipes which can be used to calculate the flow rate of the liquid in the flow pipes.
The Coriolis force meters are complicated in that they require fragile and customized plumbing components, sensitive electronic movement detectors or fiber optic cables, power supplies and light sources.